with the Younger Dryas climatic reversal that occurred during the Last Glacial Maximum (LGM)-to-Holocene transition (e.g., Broccker and Denton, 1989, and see Broecker, Chapter 5 in this volume). High-latitude foraminifcra-based sea surface salinity (SSS) reconstructions by Duplessy et al. (1991), and others subsequently, have indeed established that high-latitude Atlantic SSS was strongly depressed during glacial conditions in the regions where North Atlantic deep water (NADW) forms today (see Chapter 14 by Duplessy in this volume). An important recommendation for future research is that more-detailed analysis with more fully anticulated models be performed to test the robustness of the above referenced theory of the D-O oscillation. Because the ice-mechanical instability through which Heinnch events are generated appears to be entrained to the D-O oscillation, it would appear necessary to develop a sound theory of the latter in order to better understand the details of the former.
The issue of the mcchanism(s) underlying rapid climate change is not one that we might reasonably expect to be entirely settled on the basis of models; the level of complexity involved is simply too high for us to be overly sanguine of success through this means alone. What is also required is a concerted program of data analysis, perhaps focused on the most recent 2000 years of Earth history but including further work on the YD event. In this, and on the basis of the most recent literature, it would seem important to pay particular attention to the relative phasing of events in the Northern and Southern Hemispheres (see Chapter 5 | Broecker] and Chapter 15 [Jouzel] in this volume). That an out-of-phase relationship existed during the YD now seems to be well established on the basis of the intercomparison of accurately time-constrained data from the Greenland and Vostok Antarctica ice cores. One possible explanation of such a relationship between the hemispheres posits a complementarity of the deepwater production rates between them. When NADW production rate is high, the Northern Hemisphere is warm. When this rate falls and the Northern Hemisphere is cooled in consequence, the rate of Antarctic bottom water (A ABW) production increases to compensate, and the Southern Hemisphere is thereby warmed, Broecker has suggested that support for this view is found in the 14C age of abyssal waters. In an enhanced program focused on the most recent 2000 years of Earth history, an appropriate primary target might be the so-called Little Ice Age during which there occurred a distinct maximum in the extent of mountain glaciers. Although the cost would be high for the sufficiently high frequency sampling of the Vostok core to support such a focused effort, this should nevertheless be carefully considered, w
Paleoclimate inferences also continue to provide important constraints on the mechanics of the global carbon cycle. In this regard, however, a significant further gap in understanding continues to be that related to the mechanism(s) responsible for the glacial-to-interglacial variation of atmospheric carbon dioxide concentration. The recently constructed models of the 100 kvr ice-age cycle, which are successful in explaining this phenomenon as a nonlinear response to orbital insolation forcing (Tarasov and Peltier, 1997, 1999), employ the Vostok measurements of the variation of atmospheric CO2 concentration from Eemean to Holocene to enhance the insolation signal and strongly suggest that the observed |CO>| depression during the glacial period is
PALEOCLIMATIC CHANGE AM) GREENHOUSE IMPLICATIONS
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